The present invention relates to an amplifier circuit as defined in the preamble of claim 1, and to a method for reducing stray feedback as defined in the preamble of claim 9. In particular, the present invention can be applied in a trans-impedance amplifier provided in a reproducing device for reproducing a record carrier such as an optical disc.
Transimpedance amplifiers are commonly used in instrumentation and communication applications to provide signal gain with minimal added noise, thus maintaining a high signal-to-noise ratio. The smaller the desired signal which is applied to the amplifier, the more critical it is that the signal-to-noise ratio is properly maintained.
In reproducing devices or players for optical record carriers, transimpedance amplifiers (TIA) are used for amplifying a photo diode current. The amplified signal is then coupled to a signal processor via a flexible cable or the like. Thereby, stray currents are induced at or coupled to the input of the TIA by stray feedback, e.g. capacitive feedback. This may cause an attenuation of high frequencies, or instabilities due to an additional phase shift. The amount of this stray feedback depends on the stray capacity between the output terminal and the input terminal, and the gain of the TIA. The stray capacity is determined by the wiring area, wherein the surface area of the output wiring is generally large due to the flexible cable. Furthermore, the wiring area of the input connections is large if discrete photo diodes are used. However, the wiring area of the input connections can be reduced if a PDIC (Photo Diode IC) is used, in which photo diodes are integrated with the TIA. The required gain or transimpedance ZT of the TIA is determined by the radiation power or light intensity used, the reflectivity and the modulation depth of the record carrier, and the efficiency of the radiation path.
The above problem plays a major role for record carriers having a low reflectivity, e.g. re-writable Compact Discs (CDRW) or re-writable Digital Versatile Discs (DVD+RW), and for non-polarizing radiation paths with low efficiency in both directions.
The allowed value of the stray capacity Cs can be calculated by using the following equation:                                                                                          f                            ⁢                              -                3                                      ⁢                          xe2x80x83                        ⁢            d            ⁢                          xe2x80x83                        ⁢            B                    =                                                    1                                  2                  ⁢                                      π                    ·                                          Z                      T                                        ·                                          C                      S                                                                                  ⇒                              C                S                                      =                                          1                                  2                  ⁢                                      π                    ·                                          Z                      T                                        ·                                                                                           f                                            ⁢                                              -                        3                                                                              ⁢                                      xe2x80x83                                    ⁢                  d                  ⁢                                      xe2x80x83                                    ⁢                  B                                            =                                                                    1                                          2                      ⁢                                              π                        ·                                                  10                          5                                                ·                        1.5                        ·                                                  10                          8                                                                                                      ⁢                  F                                ≈                0                                                    ,                  01          ⁢                      xe2x80x83                    ⁢          p          ⁢                      xe2x80x83                    ⁢          F                                    (        1        )            
wherein ZT denotes the required transimpedance (gain) of the transimpedance amplifier circuit in case a typical photo diode current IPD of 10 xcexcA and a typical single-ended decoder voltage (output voltage of the transimpedance amplifier) of 1 Vpp are assumed. fxe2x88x923dB denotes the critical frequency (bandwidth) of the input signal in case of a DVD-ROM (Digital Versatile Disc Read-Only-Memory) player.
However, the thus obtained required value of the stray capacitance Cs of 0,01 pF is unrealistically low. Realistic values have to be greater by a factor 10 to 30. A reduction of the transimpedance or gain would reduce capacitive feedback, but would cause a higher noise sensitivity. Moreover, an extra amplifier would be required at the end of the flexible cable to provide a sufficient signal level.
An attempt to solve the above problem is to use a differential output amplifier circuit as shown in FIG. 5. In FIG. 5, a transimpedance amplifier 20 is shown which comprises a differential output having an inverting output terminal (out of phase) RFN and a non-inverting output terminal (in phase) RFP. The inverting output terminal RFN presents signals 180 degrees out of phase with respect to the non-inverting output terminal RFP. The differential amplifier circuit 20 can be formed using either bipolar technology or FET technology.
Furthermore, stray capacities 11 and 12 are provided between the non-inverting output terminal RFP and the inverting output terminal RFN, respectively, such that respective feedback currents are fed back to the input terminal and added to the photo diode current IPD. Due to the opposite phases of the respective feedback currents or signals, their values are subtracted at the input terminal of the transimpedance amplifier 20, whereby the resultant capacitive feedback is reduced.
However, as a result of asymmetries in the construction of the transimpedance amplifier 20, a netto stray capacitance remains, such that a certain level of stray feedback current remains at the input of the transimpedance amplifier 20.
It is an object of the present invention to provide an amplifier circuit and a method for further reducing the resultant stray feedback.
This object is achieved by an amplifier circuit as defined in claim 1, and by a method for reducing stray feedback as defined in claim 9.
Accordingly, the resultant capacity feedback is further reduced by the feedback compensation terminal as an additional output terminal for providing the predetermined fraction of the output signal which is adjusted in such a manner that the resultant stray feedback is minimized.
Preferably, the output terminal is a differential output terminal comprising a first and a second output terminal, and the output signal is a differential output signal. Thus, the differential output formed by the first and second output terminals already strongly reduces stray feedback interference, wherein the third feedback compensation terminal enables a further reduction of remaining asymmetries by properly adjusting the gain.
According to an advantageous development, an adjustable voltage dividing means may be provided for generating the fraction of the output signal.
The amplifier circuit may be a transimpedance amplifier and may be provided in a reproducing device having a reproducing element for generating the input signal. In particular, the reproducing device may me an optical disc player and the reproducing element may be a photo diode.
According to a further advantageous development, the value of the predetermined fraction may be updated during a start-up or operation initiation of a device in which the amplifier circuit is provided. Alternatively, the value of the predetermined fraction may be determined and adjusted when the amplifier circuit is manufactured or assembled.
The amplifier circuit may be provided in a reproducing device, such as an optical disk player, comprising a reproducing element for generating the input signal, channel decoding means and/or error correction means coupled to the output of the amplifier circuit.